This invention relates to telecommunications networks, and in particular to arrangements and methods for adapting traffic in such networks.
Traditionally, two types of legacy telecommunication networks have been developed. The first type is connection oriented and is used for the transport of narrow band voice traffic, typically carried in TDM frames. Such networks comprise for example synchronous or plesiochronous networks. The second type of legacy network is connection less in nature and is used for the transport of broad band packet or cell-based data traffic. There is currently a drive towards unified networks which provide end to end transport for both voice and data services. However, as there is a well established voice network base, network operators are naturally reluctant to replace such legacy networks. This issue has been addressed by providing broad band (asynchronous) overlay networks which interface with the established TDM networks to provide a voice and data transport function. At the interface between the two networks, an interface function maps TDM frames into packets or ATM cells and vice-versa. ATM is of course just one example of a packet based network.
A particular problem with the introduction of ATM transport networks is that of interfacing or inter-working with existing legacy networks which can carry many different types of traffic including voice, data and IP (Internet protocol) traffic. These services are accommodated by different ATM adaptation layers and thus require different adaptation processors to perform the process. The adaptation process is generally known as a trunking function that provides an interface between an ATM network and a non-ATM network whereby the end-to-end ATM network users have no visibility of the presence of the interconnecting ATM network. Alternatively, this process can be a co-adaptation process generally known as an interworking function that provides an interface in the ATM domain to re-adapt the services between different ATM adaptation layers, for example AAL1 to AAL2 etc.
As discussed above, a number of standards define the various adaptation layers (AALs) that are used to adapt the traffic to the appropriate ATM format. In particular, the following adaptation layers are currently in use.
AAL0 is defined as having a forty eight byte traffic payload per VC and incorporates no sequence numbering or content protection. The payload is not structured. Typically, this adaptation layer is used for continuous bit rate services which do not need to be secured and as an internal transport mechanism for carrying telephony control/signalling (CAS) to a signalling engine to process the data in a pseudo-structured fashion of which the adaptation layer itself is unaware.
AAL1 has a forty six or forty seven byte traffic payload structure depending on whether it is of structured data transfer (SDT) or of unstructured data transfer (UDT) The first byte at the beginning of the payload sequence is used for sequence numbering and the second byte, if it is structured, is used as a pointer once in every modular eight cycle to signal the start of a data structure in the payload. This latter process is optional depending on the data structure being carried in the payload. The structured mode of operation within the ML (SDT, P-format) is such that one or many time division multiplex (TDM) channels are adapted together to form a constant bit rate stream of AAL1 SDT cells in every TDM frame period. As for the unstructured mode (UDT, non-P format) this adaptation is used for continuous bit rate services which do not have an explicit data structure within the AAL1 UDT cell streams. In this application, either single TDM telephony channels or multiples of such channels are organised on to a single connection, and it is left to the end-to-end termination points to frame/re-frame in order to recover these data channels.
The bit order in the AAL1 UDT non-P-format is still preserved but alignment of the internal byte structure of data may well precess against the ATM byte structure. For example, the standard. T1 unstructured format comprises one hundred and ninety three bits which will not divide exactly by eight to fit into the byte structure form of the ATM cell. This unstructured mode is typically used for circuit emulation services where the traffic source is not synchronised to the ATM network (nor necessarily the PSTN) but end to end synchronisation of the data rate through the ATM network is required. It is expected that the destination recovers the framing information form the emulated stream to recover the T1 data. In structured transport, the traffic is already byte oriented so that both bit and byte order are preserved between the source and destination ends of the connection. This mode is used for single 64 kbits circuits (or subrate services carried as 64 kbits services) or mutliples of 64 kbits such as 2 (ISDN Wideband), 6 (H0-multimedia wideband call), 24 (T1 but synchronous only the 192 traffic bits carried) and 30/31 (E1 synchronous. The frame alignment word TS is not transported end to end as the frame terminates at the input to the adaptation layer).
The structured transport method is not dynamic. The structured VC is set up for the duration of the call and broken down afterward but cannot be changed during the call
AAL2 is defined as having a forty seven byte payload structure. The first byte in the payload structure is used for integrity checking, sequence numbering and cell delineation. The rest of the remaining bytes of the payload contains mini-packets (CPS packets) each with their own packet headers. AAL2 is usually used for delay sensitive variable bit rate services such as voice and image data services. For example either single 64 kbits telephony channels, subrate channels ( less than 64 kbits such as ADPCM or channels with speech silence removal features) or multiples of such corresponding groups of channels can each be adapted into their corresponding mini-packets which are then multiplexed into a single virtual connection(VC). The VC in this case can be deemed a variable rate pipe as connections within the pipe can be resized dynamically, started and ended whilst the VC is continually active. The connections in this VC are mini-packet connections where each circuit is identified uniquely by the combination of the circuit identifier (CID) and the VC number which carries that CID mini-packet. Mini-packet connections are connections in their own right but to the standard ATM network this variable rate feature and easy path set-up/removal makes the VC carrying the mini-packet appear as a VC with variable width. There are secure message transfer mini-packet types available for messaging i.e. messages protected by a CRC header or a trailer.
AAL5 uses a full a forty eight byte payload structure available in an ATM cell, but he adaptation method does provide message integrity protection over a block of AAL5 cells that comprise a message block via the CRC-32 bytes residing within the message trailer itself. The services of an AAL5 connection are generally message based and hence a variable bit rate service rather than continuous bit rate, although continuous bit rate services are supported. AAL5 data (or voice) is protected by CRC characters at the end of each message. Generally, ML5 services have the lowest priority in the ATM switching network.
At present, adaptation of the above described typical traffic services to these various adaptation layers requires fully separate adaptation processing for each layer. This is expensive in terms of equipment, adaptability and ownership by the network administration due to the difficulty of forecasting the nature and volume of the particular services.
A further disadvantage of current adaptation circuits is the difficulty of providing a multiple adaptation layer capability that is scalable and embodies a large connection capacity in an integrated circuit single chip structure. The respective demands on silicon area of the common part sublayer and of the service specific convergence sublayers on a common monolithic substrate can place severe constraints on the total traffic handling capacity of the device.
An object of the invention is to minimise or to overcome the above disadvantages.
A further object of the invention is to provide an improved arrangement and method for typical ATM adaptation of communications traffic.
According to a first aspect of the invention there is provided an asynchronous transfer mode adaptation processor comprising a set or suite of integrated circuit devices and being partitioned by AAL functions into a first device or device set arranged to perform a common part sublayer function and a second device or device set arranged to perform service specific sublayer functions.
According to a further aspect of the invention, there is provided an interface arrangement for providing interworking between packet (IP), time division multiplex (TDM) and asynchronous (ATM) networks, the interface comprising TDM framing means providing an interface to the TDM network, packet framing means providing an interface to the packet network, and ATM adaptation means providing an interface to the ATM network, wherein said ATM adaptation means comprises a set or suite of integrated circuit devices and being partitioned by AAL functions into a first device or device set arranged to perform a common part sublayer function and a second device or device set arranged to perform service specific sublayer functions.
According to a further aspect of the invention, there is provided an interface arrangement for inter-working of traffic between a first TDM network, a second Internet Protocol (IP) network, and an ATM network, the interface comprising;
a TDM framing circuit providing an interface to the TDM network;
an IP packet framing circuit providing an interface to the IP network;
an ATM adaptation processor providing an interface to the ATM network; and
a codec providing a coupling between the TDM framing circuit, the IP packet framing circuit and the ATM adaptation processor;
wherein the interface arrangement is such that such that data traffic is passed, directly between the TDM framing circuit, the adaptation processor and the IP framing circuit, and voice traffic is passed indirectly between the TDM framing circuit, the adaptation processor and the IP framing circuit via the codec
According to another aspect of the invention, there is provided a method of providing interworking between packet (IP), time division multiplex (TDM) and asynchronous (ATM) networks via an interface comprising TDM framing means providing an interface to the TDM network, packet framing means providing an interface to the packet network, and ATM adaptation means providing an interface to the ATM network, wherein said method comprises partitioning said adaptation means by AAL functions into a first device or device set arranged to perform a common part sublayer function and a second device or device set arranged to perform service specific sublayer functions.
By choosing the constituent devices or chips based on the service demands and the network capacities, the chip-suite can be configured to support large scalable connections of Adaptation Layer""s AAL-0, AAL-1, AAL-2 and AAL-5 for voice/data/messages, in the ATM trunking, interworking or AAL-2 switching system applications. The arrangement provides a functional partitioning of devices that can be optimised for variable and fixed packet adaptation layers having a high degree of flexibility in isolation or in combination to serve trunking, interworking, and switching of the said adaptation layers. Further, the functional partitioning can be optimised to provide significant scalability.
The functional partitioning of devices facilitates separation of concerns for traffic management, Quality of Service (QoS) controls, buffer depth scaling and low latency, and provides for integration and interworking of ML based and IP based traffic.
Large connection capacity can be accommodated such that a large AAL-2 adaptation layer switch, carrying pre-compressed voice and clear data CPS Packets via the wide-bandwidth (such as OC-12 or similar) ATM switch network interfaces, can be constructed by simply re-using the necessary constituent chips of the chip suite. Key AAL-2 specific SSCS""s SDU processing functions (e.g. UUI termination, rate profiling, PDV dejitter and plesiochronous compensation) are advantageously supported within the chip suite to minimise external digital signal processor (DSP) or uP MIP requirements. For large connection capacity such as 8K or more this external MIP minimisation is significant.
Other standard transport media such as Frame-Relay or T*/E* can be used instead of ATM for the network interface as long as it has the capability and link bandwidth to transport encapsulated ATM cells for the required traffic and control information.
By choosing constituent chips based on the service demands and the network capacities, the chip-suite can be configured to support large scalable connections of Adaptation Layer""s AAL-0, AAL-1, AAL-2 and AAL-5 for voice/data/messages, in the ATM trunking, interworking or AAL-2 switching system applications.